Dr. Sudhanshu Barway is an Assistant Professor at the Indian Institute of Astrophysics, Bangalore. An observational astronomer from the Stellar and Galactic Astronomy Research Division, Dr. Barway is interested in data-driven astronomy and big data. He has previously worked at the South African Astronomical Observatory as a staff astronomer. This conversation with him provides us insight into one of the interesting recent discoveries at IIA.

• Thank you so much for being here with us today. To begin, could you tell us a little about your area of research?

I’m mostly interested in extragalactic astronomy and I study nearby galaxies – so I would like to know how they evolve – how they were formed, and how their individual components like the central structure (the bulge) and the outer region (the disk) co-evolve with the evolution of the galaxy as a whole. Thus, I study how a galaxy transforms – basically how it changes its shape and size. A galaxy is formed either in an isolated environment, or in a group (a cluster), where it undergoes mergers and changes its shape. I’m interested in such morphological transformations and of course the supermassive blackholes that we see at the center of almost all galaxies.

• Astronomy and Astrophysics is a field which has always had that “wow” factor around it. What excites you most about your work?

There are two things, actually. The first is that I’m an observational astronomer. I have been one all my life. I observe and analyze the universe with telescopes, [with apertures ranging between about] six to eight inches to as big as eight metres. The second is the data science involved in astronomy. There are observatories not only on the ground but also in space like the James Webb Space Telescope. The thing with large telescopes (like the ones greater than 8 inches) is that you cannot go and observe firsthand, someone will go and observe for you and then give you the data. There is such a large amount of data available and I find working with this data to be very interesting.

• Coming to your latest discovery, we would really love to get a firsthand account of it. What exactly was observed and how did you go about understanding it?

Okay, so let’s start at the beginning – which is actually quite a few years back! Almost four or five years ago, when I was working at the South African Astronomical Observatory (SAAO) as a staff astronomer and visiting IIA Bangalore, I interacted with the staff here who were working in extragalactic astronomy and we came up with a common project of observing merging galaxies. My collaborator Dr. Mousumi Das and I decided that since I worked at SAAO which has access to a large variety of telescopes, we would work on mergers of two or more galaxies, observe such galaxies in optical and near-infrared wavelengths, and study the various properties of these merging galaxies.

So in 2017, we observed samples of some 10 galaxies and got data in the near-infrared band which tells us about the old stellar population in the galaxies. Since we were looking for mergers, which tend to produce a lot of gas that can help us look for star formations, we decided to observe these galaxies in the ultraviolet band in 2019. Looking at archived data from an 8 metre telescope in Chile, we obtained image slices per wavelength. The combination of these three gave us a lot of information about our sample.

In our sample, we were looking for a merger of the interacting galaxy pair NGC7733N and NGC7734. Using the three sets of data, we detected unusual emissions from the centers of these two galaxies. This emission told us that the centers can be classified as AGNs – Active Galactic Nuclei – which basically means that there is the presence of a supermassive black hole in the center.

What happened next was very interesting. We found a bright clump along the northern arm of the NGC7733N. Initially we thought that it might be a very big star forming clump, which is pretty common in merging galaxies, as during the process of merging, the gas present in the galaxy gets compressed and can start forming new stars. However, we had information on the velocities from something called an Integral Field Spectrograph (IFS), which we use to measure the redshift of these galaxies. If the clumps belong to the same galaxy, their redshifts also have to be similar - but for this particular clump we observed the redshift to be different! This difference was large enough to imply that the clump was a different galaxy altogether.

Another interesting observation we made was that this third galaxy also contained a supermassive black hole. Merging supermassive black hole pairs are known to occur commonly, but this was a rare case in which we had not just a pair but a merging group of galaxies, all of which had a black hole at the center.

• Let’s talk about the day this discovery was made. How was the atmosphere in the lab? Who all were involved in this study and what were their first reactions?

Haha, okay! The data that we used was gathered over the course of four or five years, which is a pretty long time. So once we found out the anomaly, the first thought that came to our minds was “Wow, this looks pretty rare!” and the next one was “Okay, but we need to be absolutely sure.” We again went through our analyses to recheck everything once over to verify [our discovery].

There weren’t many people around at the moment, just the three of us. Moreover, a major part of this was during the [Covid-19] lockdown, so we were a small group of three people constantly working online on our computers sitting at home. Unfortunately we did not get to celebrate in person but we definitely celebrated online.

• What were some of the biggest challenges you faced over the course of this study?

I would say the main challenge was to get a good data set, and once that was covered, the next challenge was to perform a thorough analysis. Whatever results we are obtaining from this analysis should be very precise, so that we can effectively explain the observations in terms of physics. The ultimate goal is always to understand what are the physical processes behind all these astrophysical phenomena.

• Is a galactic merger supposed to be a common event? How do these systems behave? How do we get information about these black holes?

The merging phenomenon is quite common, there are no surprises there. Mergers are very long lasting processes. They are slow enough that if one is in the merging system, they won’t feel the merging happen, but an outside observer could detect that the galaxies are going through a merger. For example, our own Milky Way and the Andromeda are going to merge in the next five billion years! They will quietly pass through each other, merge and form a common system. There are also different types of mergers, based on the orientations of the galaxies – head-on collisions, side-on collisions, rotation about a common center of mass and so on.

Most galaxies have a supermassive black hole, but detecting them is a challenge, since black holes don’t emit light. Luckily, their presence can be revealed by their interaction with their surroundings. If there are any gases or dust in the proximity of the supermassive black hole, some of it is swallowed by the black hole but some actually gets converted into energy and emitted as electromagnetic radiation. That makes the black hole appear luminous, and this is where the term “Active Galactic Nuclei” comes from. All of these three galaxies have an AGN, which is a supermassive black hole emitting luminous electromagnetic radiation.

• We read that you used data from the UV Imaging Telescope aboard the ASTROSAT and the VLT in Chile. What is the nature of this data and how is it processed?

We used three telescopes each of which observed in different regions of the electromagnetic spectrum. One observed in near-infrared which gave us information about old stars. When stars grow old and their emissions become relatively less hot, they emit [most] in the infrared region. During mergers, gas compression can lead to star formation which can be looked at in the ultraviolet (UV) region.

What we also need to be looking for is various abundances – what kind of elements are present, stellar population histories, and also the distribution of star formation along the different components. The IFS instrument, MUSE, gave us this data. Two important things we gleaned from this data were the red shift information, and the existence of the supermassive black holes at the centers.

There is a method of measuring different line ratios [between spectral lines] such as H-alpha from hydrogen’s Balmer series depending on the galaxy’s wavelength. From the rest frame, because the galaxy is moving away from us, the lines will also be shifted accordingly. Comparing the observed wavelength with the laboratory wavelength, we obtain the ‘rest-frame wavelength’. So we observed ratios of H-alpha, H-beta, nitrogen, oxygen and other such lines – and these ratios tell us whether the center of the galaxy is active or not. If it is active, it indirectly implies the presence of the black hole.

• What are the implications of your discovery for the galaxies that were involved? How does this observation fuel further research?

Mergers of galaxies that host AGNs are very common. But finding an interacting galaxy group like we observed is very rare. In fact, if I remember correctly this might be the third such case. Obviously, at the moment we can’t really tell what will happen when these galaxies merge because it might take a billion years. But this group does present a new approach to the subject. The dynamics of this system is a very intriguing potential area of research. How the presence of the third black hole will play a role in the processes of the two bigger black holes is something we would like to study in greater detail. So thanks to this event, we stand to observe, analyze and understand much more complex models of the merging phenomenon.

• What are some of the common prevalent methods astrophysicists use to study galaxies and related celestial bodies, apart from what we already discussed today?

The work that we did here is a combination of our observations and the data that we gathered from the archive. As we saw, it takes a long time to obtain, process and analyze such data. Another situation that you can have is, say you don’t have access to a telescope. That doesn’t mean that you can’t work on observational astronomy. At the end of observations, we get different types of data – data in terms of images, spectra, and catalogues. When we talk about astronomical data science, we actually provide ‘science-ready’ data, which means that you don’t need to go for observations, you just need to go to the data archive and download the data from there.

At the start of every new research problem the first thing we question is whether this can be done using archival data, which is open access to all; one doesn’t need to pay for it. Our third data set from the 8 metre VLT in Chile was similar to this. Someone else makes these observations. They are given a year to process and analyze it for their own work, after which it is published. Our ASTROSAT data for this galaxy is also now public - anyone can access and use it.

• Can you talk to us about the post-publishing scenario? What does the publication and peer review process look like?

Once you have a discovery or any research that you want to document and share, you write the paper in a certain format depending on what journal you are aiming to publish it in. Once you submit a manuscript to a journal, it goes through a referring process – the journals ask for experts to review your work. Two or three referees examine your work and present their views and suggestions, and ask questions based on it. This process goes on in rounds with modifications at every round until the referee is happy with the work. Through this process the publication is also improved, as more questions are answered, enriching the publication. Once this process is done, the journal accepts your paper for publication, which can subsequently get more peer reviews and citations.

• What does the research group involved in this discovery have in store for the future? Any exciting research that you might want to share?

We are always looking for ways to continue and add on to our work. It can be extended to look at other aspects of this group of galaxies, and it would be interesting to see how the information we gained can be used in the context of some other work.

Actually, another one of the projects I am working on is providing the data from the various IIA telescopes to some interfaces. Basically it’s like a google search but specific towards any observatory. This database will take search queries about astronomical events, and provide information such as which instrument was used for observation and whether science-ready images are available or not.

This has been a very fascinating ride, and more than a little enriching. Thank you very much for sparing time from your busy schedule for interacting with us!

//Include figure with article:

Ref: Yadav, Jyoti & Das, Mousumi & Barway, Sudhanshu & Combes, Francoise. (2021). A Triple AGN in the NGC 7733-7734 Merging Group. Astronomy and Astrophysics.

Note

Dr. Barway has been working with the Pune Knowledge Cluster on a Citizen Science initiative - finding features in galaxies. Not requiring any prior knowledge of astronomy or astrophysics, it’s a good idea for us as citizens to participate in this project and help astronomers classify various features in galaxies.

The participation manual is appended herewith:

PKC_CitizenScience_participation_manual.pdf

Contact Chrysalis for more details, and please write to PKC on citizen.science@pkc.org.in for queries/assistance.